Russia take capital??? Maybe, but I consider it unlikely. Sarajevo was sieged for almost four years. If there are supplies coming in the city, it can defend almost indefinitely. Plus Kiev is HUUGE. Encircling it would be very hard, taking even harder. Even then, breaches of encirclement to provide supplies are possible. Note, that as the war goes on, Ukrainian manpower will grow. They are in full mobilization, but it takes time. Foreign fighter will keep coming.
Russian manpower will get worse and they can not enter total war and full mobilization (which is what Ukraine will be doing), as it would cause unrest in the country. As the Russian army is bogged down in Ukraine, they will have to divert resources. If they withdraw from Syria, Assad might fall. They need to keep resource in the country to prevent unrest and rebellion. As economy collapses, you can bet that Dagestan and Tatarstan will have ideas about independence, Chechens likely too. Who knows who else. If the war keeps long enough, Russia might end up losing their own territory.
Also while Russian economy and war effort collapses under these sanctions, Ukraine will be propped by the west. Putin is right in one thing, these sanctions are a declaration of war. He isn't fighting just Ukraine, he is fighting whole western world. Ukraine is just a proxy. Unless China bails him out (which I don't see as they will want to stay neutral) he can't keep this up. He needs to finish this and quickly. But I don't see how.
Unfortunately, war toll will be heavy on all of us and it will fall hardest on Ukraine.